Fuel fleas

Fuel fleas are microscopic hot particles of new or spent nuclear fuel. While small, they tend to be intensely radioactive.

The fuel particles, the size about 10 micrometers, are a strong source of beta and gamma radiation and a weaker source of alpha radiation. The disparity between alpha and beta radiation (alpha activity is typically 100-1000 times weaker than beta, so the particle loses much more negative-charged particles than positive-charged ones) leads to buildup of positive electrostatic charge on the particle, causing the particle to "jump" from surface to surface and easily become airborne.

Fuel fleas are typically rich in uranium 238, and contain an abundance of insoluble fission products. Due to their high beta activity, they can be detected by a Geiger counter. Their gamma output can allow analysis of their isotope composition (and therefore their age and origin) by a gamma ray spectrometer.

Fuel fleas can be very dangerous if they become embedded within a person's body, but are generally not considered more dangerous than an equal amount of radioactive material evenly distributed throughout the body. An exception would be if the flea was embedded in a particularly vulnerable organ, such as the cornea of the eye, inhaled into the lungs.

When the fuel pellets are not thoroughly dried during manufacturing, the excess moisture reacts with the hot metal and releases hydrogen, which enters the lattice of the zirconium metal of the cladding of the fuel rod. The resulting hydrogen embrittlement leads to formation of microscopic holes in the cladding, through which the fuel particles can escape and through which the cooling water can enter the fuel rod, further accelerating the process.